1. Field of the Invention
The present invention relates to a mobile station, mobile communication system, handover control method, handover control program and recording medium.
2. Related Background Art
In mobile communication systems such as those of mobile telephones which are currently commonplace, service is provided with the entire service area divided into comparatively small zones called cells. Such a system is constituted by a plurality of base stations that cover the plurality of divided wireless zones and mobile stations that perform communication by setting up wireless channels with these base stations.
Electromagnetic waves that are transmitted with a given transmission power from the base stations are attenuated as they are propagated through space until they arrive at the reception point. Since electromagnetic waves have the property that their attenuation increases as the distance between the transmission point and reception point increases, essentially pilot channels that are transmitted from distant base stations are received with a weak reception level while pilot channels that are transmitted from near base stations are received with a strong reception level. In fact, the magnitude of propagation loss depends not only on distance but also on circumstances such as topography or buildings, so the received power of the pilot channels from various base stations fluctuates considerably with movement of a mobile station. In order to receive the signal transmitted from the base stations with better quality, it is important for the mobile station to constantly monitor the pilot channels from the various base stations and to select the optimum base station. By applying such control, the base station that is evaluated as optimum successively changes every time the base station is moved, so the base station that is connected with the mobile station also successively changes. The control of successive changeover of the base stations that are connected as movement takes place is termed “handover control”.
Handover control may be “hard handover” or “soft handover”. In case of hard handover, the number of base stations which the base station is connected to is always one, and the base station which the mobile station is connected to is changed over as movement takes place. In contrast, soft handover is a method of control whereby the mobile station can be connected with a plurality of base stations, the mobile station being connected with at least one base station and, depending on circumstances, with a plurality of base stations. In this handover, the base station that is connected with the mobile station is added or deleted, tracking the movement of the mobile station. “Diversity handover” is a type of soft handover. Diversity handover is characterized in that signals transmitted from a plurality of base stations can be received in combined fashion by a mobile station or signals transmitted from a mobile station can be received and combined at a plurality of base stations, so reception quality can be improved by the diversity effect.
As a method of exchanging the so-called packet data between base stations and mobile stations, in particular a method of exchanging packet data by a downlink circuit from base stations in the direction of mobile stations, the method is sometimes adopted of multiplexing and transmitting data of a plurality of the mobile stations on a single or a plurality of channels shared by a plurality of mobile stations. As the method of this multiplexing, time division multiplexing or code division multiplexing or frequency division multiplexing etc may be employed and, in addition, the scheduling of data transmission to mobile stations of different types can be efficiently achieved by various types of algorithm at the base station end. When such shared channels are employed, the various base stations typically use dedicated channels as well for notification of the timing of data transmission to the mobile stations or control of transmission power, or transmitting an uplink (transmission from the mobile station to the base station) data arrival confirmation signal. Usually, for such dedicated channels, channels are prepared that are of comparatively low bit rate so that they have little effect on the overall capacity etc. For example, in the W-CDMA (Wideband Code Division Multiple Access) system whose development is proceeding as a third generation mobile communication system and which is being provided as a service in Japan from May 2001. In the standards provided by the 3GPP (Third Generation Partnership Project), a PDSCH (Physical Downlink Shared Channel) is specified as a channel (physical channel) shared by a plurality of users and an A-DPCH (Associated Dedicated Physical Channel) is specified as a channel for exclusive use by each user in conjunction therewith.